Wireless networks based on the IEEE 802.11b standard are increasingly deployed in corporate enterprises and public hot spots, such as airports, hotels and conference facilities. Generally, wireless networks based on the IEEE 802.11b standard are used primarily to provide wireless data access from portable computing devices, such as portable computers and personal digital assistants (PDAs), to a wired network, such as an enterprise network or the Internet. Increasingly, wireless networks based on the IEEE 802.11b standard are being used to carry voice traffic, typically referred to as “voice over IP” (VoIP) traffic.
The IEEE 802.11b standard has a large fixed overhead. Thus, the bandwidth available at the payload sizes typical for VoIP traffic is far less than the bandwidth available when using the network for data traffic. The 802.11b standard currently supports data rates of 1, 2, 5.5 and 11 Mbps. When sending data frames with the maximal data rate of 11 Mbps in such an IEEE 802.11b network, the maximal achievable throughput is approximately 6.2 Mbps. When sending frames for VoIP traffic, however, the maximal achievable throughput is only approximately 2 Mbps, for a typical audio payload size for a packet in a Real Time Protocol (RTP) audio stream.
The significant difference between the achievable throughputs for audio traffic and VoIP traffic is primarily due to the large transmission overhead per frame, regardless of the frame size. Depending on the actual average transmission rate, the number of simultaneous VoIP calls in a cell (the term “cell” is used to refer to the Basic Service Set (BSS) using terminology of the 802.11 standard) of the wireless network is between 4 and 17 calls for an exemplary G711 codec with 30 milliseconds of audio data per packet.
If a wireless network based on the IEEE 802.11b standard accepts one additional call or one additional data connection that exceeds the capacity of the wireless network, an unacceptable call quality for all ongoing VoIP calls results. Furthermore, if the load offered to the network is higher than the capacity of the network, the Distributed Coordination Function (DCF) medium access scheme of the IEEE 802.11b standard curtails the client with the highest load first. In most cases, the access point of the wireless cell provides more traffic to the network than the associated stations. Hence, the access point gets curtailed first which leads to unacceptable packet loss for all VoIP streams transmitted from the access point to a client resulting in poor call quality for all connections. Thus, a need exists for a technique for measuring network capacity and for providing admission control in a wireless network carrying VoIP traffic.